include/grub/file.h: Add device tree file type

The API change of grub_file_open() for adding verifiers did not include
a type for device tree blobs. Add GRUB_FILE_TYPE_DEVICE_TREE_IMAGE to
the grub_file_type enum.

Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

include/grub/file.h

@@ -69,6 +69,8 @@ enum grub_file_type
 
     GRUB_FILE_TYPE_EFI_CHAINLOADED_IMAGE,
 
+    GRUB_FILE_TYPE_DEVICE_TREE_IMAGE,
+
     /* File holding signature.  */
     GRUB_FILE_TYPE_SIGNATURE,
     /* File holding public key to verify signature once.  */